1. Field of the Invention
This invention relates to a directional atomic force microscope and a method of observing a sample with the microscope.
2. Description of the Prior Art
The atomic force microscope (Binnig, Quate and Gerber, Phys. Rev. Lett. 12, 930, 1986) is a new type of microscope which images minute irregularities on the surface of a sample utilizing the deflection induced in a cantilever supporting a probe owing to the force acting between the sample surface and the probe. Using this atomic force microscope, Martin et al. developed a method for detecting the attractive force of a sample from the change in the resonant frequency of the cantilever when it is imparted with longitudinal vibration (Y. Martin, C. C. Williams, H. K. Wickramasinghe: J. Appl. Phys., 61(1987) 4723). On the other hand, Maivald et al. and Radmacher et al. developed a vibration atomic force microscope and measured the viscoelasticity of a sample from the vibration response of the cantilever when the sample was vibrated vertically. (P. Maivald, H. J. Butt, S. A. C Gould, C. B. Prater, B. Drake, J. A. Gurley, V. B. Elings, and P. K. Hansma: Nanotechnology 2(1991)103; and M. Radmacher, R. W. Tillmann, M. Fritz, and H. E. Gaub: Science, 257(1992)1900.) Further, Takata proposed a tunneling acoustic microscope of a type which, reverse from the foregoing, detects the vibration of the sample when vibration is imparted to the probe (K. Takata, T. Hasegawa, Sumio Hozaka, Shigeyuki Hosoku, Tsutomu Komoda: Appl. Phys. Lett. 55(1989)17). This was used by Cretin et al. (B. Cretin and P. Stahl Appl. Phys. Lett 62(1993)829) for imaging internal defects.
On the other hand, the friction force microscope (Mate, McClelland, Erlandsson and Chiang, Phys. Rev., Lett., 59, 1942, 1987) images the surface of a sample based on the frictional force determined by measuring the torsion produced in a cantilever by the frictional force between the sample surface and a probe. O'Shea et al. developed a lateral vibration friction force microscope which determines frictional force by applying lateral vibration to a sample and measuring the amplitude of the torsional vibration produced by a static frictional force. (S. J. O'Shea, E. Welland. App. Phys. Lett., 61, 2240, 1992). In this case, the vibration amplitude was held to a magnitude at which slipping did not occur so as to measure only the frictional force unaffected by surface irregularities. Although not mentioned by O'Shea et al., the time response of the frictional force can be evaluated by varying the vibration frequency in the lateral vibration friction force microscope.
In the vibration atomic force microscope and the tunneling acoustic microscope, the sample is subject only to forces produced normal thereto by the vertical vibration and, therefore, only voids and other internal structures which change the apparent elasticity in the vertical direction are imaged. However, cracks, delaminated portions of a coating and the like are not displaced unless a shearing force is added to the normal force. Moreover, as is for example seen in obliquely oriented liquid crystal a large compliance is often exhibited three-dimensionally in a specific direction. The prior art vibration mode limited to the vertical direction is therefore inadequate when measurement is to be extended to such samples.
The vibration friction force microscope of O'Shea et al. measures frictional force using lateral vibration, in which case it is not possible to limit the friction to one direction by reversing the direction of the friction every half cycle. However, as Meyer et al. found using an ordinary friction force microscope, in the case of, for example, a crystal surface having continuous steps in a specific direction (G. Meyer and N. M. Amer, Appl., Phys., Lett. 57, 2089, 1990) or a crystal in which the molecules progressively overlap, the frictional force exhibits directional dependence, meaning that there are cases in which the frictional force obtained differs between the step ascending and descending directions. In such cases, the vibration friction force microscope has a problem in that it cannot detect the directional dependence of the frictional force.
This invention was accomplished in response to the foregoing circumstances and aims to overcome the aforesaid problems of the vibration atomic force microscope, tunneling acoustic microscope and vibration friction force microscope by providing a directional atomic force microscope capable of measuring frictional force in one direction, measuring shear elasticity and detecting and imaging the features of a sample which change these, of imaging internal structural features of a sample which easily deform in a specific direction, and of evaluating the viscoelastic behavior of a sample, and by providing a method of observing a sample with the atomic force microscope.